1. Field
Embodiments described herein generally relate to an electrically rewritable non-volatile semiconductor storage device.
2. Description of the Related Art
With an increasing use of a large capacity of data such as images and videos in mobile devices, the demand for NAND type flash memories is rapidly increasing. In particular, by adopting a multi-valued storage technology for storing information of 2 bits or more in a memory cell, a larger capacity of information can be stored with a small chip area.
In highly-integrated flash memories with the advancement of miniaturization of cells, at the time of a write operation on memory cells adjacent to memory cells where a data write operation ends, threshold voltage distributions that indicate data of the memory cells where the write operation is completed are affected by interference of the adjacent cells. The influence of the interference appears in the form of an increase in the width of the threshold voltage distribution. In particular, when a multi-valued storage system is adopted, the width and distance of the threshold voltage distributions are narrowly set as compared with a two-valued storage system. As a result, the interference of the adjacent cells greatly affects reliability of data. For this reason, it is necessary to execute a read operation that can suppress an influence of a change in the threshold voltage caused by the interference from the adjacent cells.